Health and exercise professionals have long recognized the wide-ranging physiological benefits of regular endurance exercise—from cardiovascular health to improved metabolic efficiency. But new research published in Scientific Reports offers compelling evidence that those benefits extend well beyond the heart and lungs. In fact, endurance training appears to “train” the immune system itself, improving both its responsiveness and its efficiency, particularly in older adults.

A collaborative study led by researchers from São Paulo State University (UNESP), Justus Liebig University in Germany, and other institutions investigated how regular endurance training influences natural killer (NK) cells, which are an essential component of the innate immune system. These cells play a frontline role in protecting the body from viral infections and cancerous growths. The researchers compared the immune cell profiles of older men with at least 20 years of endurance training experience to those of their untrained peers. Their findings offer insight into the adaptive potential of the immune system in response to long-term physical activity, and present valuable implications for those who work with aging clients.

“In a previous study, we found that obesity and a sedentary lifestyle can trigger a process of premature aging of defense cells,” says lead researcher Luciele Minuzzi, PhD, a visiting scholar at Justus Liebig University. “This made us want to investigate the other side of the story, that is, whether an older adult who has been practicing endurance exercises for more than 20 years may have a better-prepared immune system.” And, says Dr. Minuzzi, that’s what they found. In fact, they concluded that exercise doesn’t just support immune function—it fundamentally reshapes how the immune system operates.

The researchers concentrated on a specific type of immune cell called natural killer (NK) cells. These cells act as the body’s rapid response team, targeting and destroying virus-infected and abnormal cells without needing prior exposure. To see how exercise influenced these cells, the scientists studied blood samples from two groups of older men, some with more than 20 years of endurance training, and others who were not regularly active.

The NK cells were exposed to simulated stress in the lab, mimicking what might happen during infection or illness. Even under these challenging conditions, the NK cells from endurance-trained individuals performed significantly better. They responded quickly and effectively without becoming fatigued, while the cells from untrained participants showed signs of burnout and struggled to mount a proper immune response.

What’s especially interesting is that the trained individuals’ NK cells were also more energy efficient. These cells used oxygen more effectively and had greater metabolic flexibility, meaning they could adapt better to stress and keep functioning when it mattered most. As Dr. Minuzzi explains, “even with the blocking of signaling pathways, the NK cells of the trained older adults were able to maintain their immune function,” while the untrained cells showed signs of failure.

In simpler terms, years of endurance training appear to “coach” the immune system, helping it stay strong, adaptable and efficient well into older age.

These adaptations are especially relevant in the context of aging, a period typically characterized by the gradual deterioration of the immune system (the technical term for this is immunosenescence) and “inflammaging,” or chronic low-grade inflammation. Both conditions compromise immune surveillance and contribute to the onset of age-related diseases. But as co-author Fábio Santos Lira, PhD, explains, the study suggests that long-term endurance training can counteract these declines. “When we compared the cells of trained older adults with those of non-athletes of the same age, we found that those with a history of endurance exercise had fewer inflammatory markers and more anti-inflammatory markers,” says Dr. Lira. “They had much better control of inflammation.”

What This Research Means for Health and Exercise Professionals

From a practical standpoint, these results reinforce what many health and exercise professionals have observed anecdotally: active older clients tend to recover faster, respond better to vaccines and report fewer illnesses. But this study adds critical mechanistic insight, showing that endurance training reshapes immune function on a cellular and metabolic level. That matters not only for athletic longevity but also for supporting healthy aging more broadly.

So, what does this mean if you work with older adults or those approaching retirement age? First, it underscores the importance of consistency and duration in aerobic exercise. These immunological benefits were not observed in short-term or newly active individuals; rather, they emerged in those with a sustained history of endurance training. While the study involved a relatively small sample size (just nine men), its findings align with a broader body of research showing that physical activity mitigates immune aging.

Second, the study offers reassurance that exercise-induced stress—such as the temporary rise in inflammatory markers or oxidative stress during a workout—is not only tolerable but may in fact be necessary to stimulate these adaptive responses. NK cells are known to be highly responsive to catecholamines like adrenaline and noradrenaline, both of which are released during moderate to vigorous intensity exercise. Repeated exposure to these molecules appears to “educate” the immune system, enhancing its capacity for future challenges.

For clients who are already active, especially those who enjoy aerobic activities such as cycling, running, swimming or walking, this research can be used to reinforce the value of their commitment. But for older adults leading a sedentary lifestyle or who are minimally active, these findings can serve as a motivational tool: Regular endurance training has tangible, measurable effects on immune resilience. You can play a critical role in guiding your clients toward an achievable routine that emphasizes cardiovascular endurance, even if starting from scratch.

It’s also worth noting that the benefits appear to go beyond immediate immune activation. “Trained older individuals demonstrate more efficient and adaptable immunity, with greater metabolic control and less propensity for cellular exhaustion,” says Dr. Minuzzi. In practice, this translates to better vaccine responses, reduced susceptibility to infection, and potentially lower risk for chronic conditions tied to immune dysfunction.

For those designing exercise programs for aging populations, the key takeaway is not to try and turn every client into a marathon runner, but to prioritize aerobic consistency in a way that aligns with individual capacity and preference. A well-rounded endurance program that includes activities the client enjoys can be one of the most powerful tools available in the pursuit of lifelong health and vitality.

In the broader picture, this study invites you to think of physical activity not just as a way to train muscles or improve heart health, but as an intervention capable of reprogramming the immune system for the better. As we look ahead to supporting an increasingly older and more health-conscious population, this paradigm shift is both timely and essential.